Field of Technology
The present disclosure relates to crucibles and to methods of forming and using crucibles.
Description of the Background of the Technology
Light emitting diodes (LEDs) are ubiquitous in modern society: they are in traffic lights, automobile interiors, backlights in cell phones, and many other applications. The growing popularity of LEDs derives from their many advantages over incandescent and fluorescent lamps including high energy efficiency, long lifetimes, compact size, and shock resistance. Furthermore, LEDs can emit light of a precise color, which is useful for certain applications.
High-power LEDs typically use sapphire single crystals as components and substrates. To grow sapphire single crystals, Al2O3 (alumina) is melted in a crucible or die. A sapphire seed crystal of desired crystallinity is dipped into the melt and pulled or drawn out, whereupon the Al2O3 is crystallized into solid sapphire. The shape of the crucible-grown sapphire is generally determined by the geometry of the crucible.
Crucibles for growing sapphire single crystals typically are produced from molybdenum. Certain crucibles for growing sapphire single crystals can measure approximately 43 cm in diameter and approximately 51 cm in depth, with a wall thickness ranging from approximately 1.0 mm to approximately 2.5 mm. A molybdenum crucible having those dimensions would weigh no less than approximately 23 kg.
The cost to fabricate a molybdenum crucible for growing sapphire single crystals can be significant. Molybdenum is brittle at room temperature, which can make fabrication difficult. Mechanical working of molybdenum must be carried out above the ductile-brittle transition temperature of the workpiece, which can be 200° C. (400° F.) to 650° C. (1200° F.), depending on the geometry and thickness of the part being formed. Forming a molybdenum sheet can require several steps of pre-heating and deep drawing may, with intermediate cleaning/annealing and re-lubrication steps between the forming steps to avoid forming cracks. Forming processes such as press brake folding of a sheet or bending of a rod may only be possible after localized pre-heating. Even if proper heat is applied, however, forming molybdenum into the desired shape may be difficult, due to material smearing/galling, undesirable thermal expansion of the tooling, and tool wear/fatigue failure. Accordingly, the present inventor identified a need for crucibles constructed in a way that address limitations of conventional molybdenum crucibles.